A complex regulatory mechanism is under investigation in Drosophila melanogaster that is controlled by wild type alleles at two different major loci. One locus known as the maternal effect locus is at about position 65.3 in the X chromosomes. The other locus known as the modulator locus is in the right arm of chromosome 3 at about position 58.3. Wild type alleles have been identified at each locus. Basic processes are involved such as the development of certain imaginal discs and the synthesis or function of ribosomes. A working hypothesis i that the maternal effect locus functions before the first meiotic division resulting in the production of a substance in the cytoplasm of the egg. Alleles at the locus in chromosome 3 modulate the maternal effect locus. Certain alleles at each locus function during embryogenesis in conjunction with other modifiers resulting in a bobbed-like phenotype, suggesting an alteration of ribosome synthesis or function.